1. Field of the Invention
The present invention relates to a magnetoresistive element and a magnetic memory.
2. Related Art
Various types of solid-state magnetic memories have been proposed. Recently, magnetic random access memories using magnetoresistive elements showing a giant magnetic resistance effect as storage elements have been proposed. In particular, magnetic memories using ferromagnetic tunnel junction elements as magnetoresistive elements have drawn attention.
A ferromagnetic tunnel junction typically has a three-layer structure including a first ferromagnetic layer, an insulating layer, and a second ferromagnetic layer. A current flows by tunneling through the insulating layer. In this case, the junction resistance value varies in proportion to the cosine of the relative angle between the magnetization direction of the first ferromagnetic layer and the magnetization direction of the second ferromagnetic layer. Specifically, the resistance value is the lowest when the magnetization direction of the first ferromagnetic layer is parallel to the second ferromagnetic layer, and the highest when the magnetization direction of the first ferromagnetic layer is antiparallel to the magnetization direction of the second ferromagnetic layer. This is called Tunneling Magneto-Resistance (TMR) effect. For example, it is reported that the variation in resistance value caused by the TMR effect is as much as 49.7% (for example, see Appl. Phys. Lett. 77,283, 2000).
In a magnetic memory including ferromagnetic tunnel junctions in memory cells, magnetization of one ferromagnetic layer of each ferromagnetic tunnel junction is pinned to make it a reference layer, and the other ferromagnetic layer is used as a storage layer. In such a memory cell, information is stored by assigning one of the binary data items “0” and “1” to the parallel relationship between the magnetizations of the reference layer and the storage layer, and the other to the antiparallel relationship. The writing of storage information is performed by reversing the magnetization of the storage layer by utilizing a magnetic field generated by allowing a current to flow through separately provided writing wiring (bit line and word line). The reading of storage information is performed by passing a current through the ferromagnetic tunnel junction, and detecting a change in resistance value caused by the TMR effect. The magnetic memory is composed of a number of such memory cells.
Other structures of magnetic memory cells have also been proposed. For example, in one method, a switching transistor is provided to each cell, as in the case of a DRAM (Dynamic Random Access Memory), so as to select a desired cell, and periphery circuits are incorporated in the memory. In another method, a ferromagnetic tunnel junction is located at an intersection of a word line and a bit line together with a diode (for example, U.S. Pat. Nos. 5,640,343 and 5,650,958).
When high integration of a magnetic memory including memory cells having ferromagnetic tunnel junctions is sought, the size of each memory cell is decreased, and thus the size of the ferromagnetic layers constituting each ferromagnetic tunnel junction is also necessarily decreased. Generally, when the size of ferromagnetic layers is decreased, the coercive force thereof is increased. This means that the switching field is increased since the level of coercive force can be an index of the level of switching field required to reverse the magnetization.
Accordingly, a higher current would be needed to flow through the writing wiring in order to write data, thereby increasing power consumption. Therefore, to decrease the coercive force of the ferromagnetic layers used in the memory cells is an important objective in achieving practical utilization of a highly integrated magnetic memory.
A magnetic memory is expected to store information stably since it operates as a non-volatile memory. There is a parameter, thermal fluctuation constant, as an index for long and stable recording, which is generally said to be in proportion to the volume and coercive force of a ferromagnetic layer. Accordingly, if the coercive force is decreased in order to lower the power consumption, the thermal stability is also lowered, resulting in that it is no longer possible to store information for a long time. Therefore, to have a ferromagnetic tunnel junction element that has a higher thermal stability and is capable of storing information for a long time is another important objective in achieving practical utilization of a highly integrated magnetic memory.
Generally, a rectangular ferromagnetic member is used for a memory cell of a magnetic memory. However, it is known that a rectangular minute ferromagnetic member has special magnetic domains, called “edge domains” at its end portions (for example, see J. App. Phys. 81, 5,471, 1997). The reason for this is that the magnetization vectors form a rotating pattern along the short sides of the rectangle so as to lower the demagnetizing field energy. FIG. 14 shows an example of such a magnetic structure. As shown in FIG. 14, at the central portion of the magnetization region, the magnetization vectors align in accordance with the magnetic anisotropy. However, at the end portions, the magnetic vectors align in the directions different from those in the central portion.
When the magnetization of the rectangular ferromagnetic member is reversed, the edge domains grow to increase their area. There are cases where the edge domains at both short sides of the rectangle are parallel with each other, and cases where the edge domains are antiparallel with each other. In the case of the parallel relationship, the coercive force is increased.
In order to solve this problem, the use of an oval ferromagnetic member as a recording layer has been proposed. (For example, see U.S. Pat. No. 5,757,695). The technique disclosed in this document is that the occurrence of edge domains at the end portions of a rectangle, etc. is suppressed by the use of the sensitive nature of edge domains against the shape of ferromagnet, thereby achieving a single domain. With such a technique, it is possible to evenly reverse the magnetization of the entire ferromagnet, thereby decreasing the reversal field.
Further, the use of a ferromagnet having no right angles, such as a paralleogram, as a storage layer has also been proposed (for example, see JP Laid-Open Pub. No. 273337/1999). In this case, although edge domains exist, the area thereof is not so large as in the case of a rectangular ferromagnet. In addition, no intricate minute domain is formed in the process of magnetization reversal. Accordingly, it is possible to evenly reverse the magnetization, thereby decreasing the reversal field.
Furthermore, the use of a rectangle having projections at one pair of opposing corners in order to decrease the coercive force as a storage layer has also been proposed (for example, see JP Laid-Open Pub. No. 2002-280637).
Moreover, the use of a multi-layer structure composed of at least two ferromagnetic layers with a nonmagnetic layer being located between the ferromagnetic layers, and with antiferromagnetic coupling existing between the ferromagnetic layers, has also been proposed (for example, see JP Laid-Open Pub. No. 251621/1997, JP Laid-Open Pub. No. 2001-156358, and U.S. Pat. No. 5,953,248). In this case, the two ferromagnetic layers have different magnetic moments or thicknesses, and have opposite-direction magnetizations due to antiferromagnetic coupling. As a result, the magnetizations are cancelled out, and as a whole, the storage layer can be deemed to be a ferromagnet having small magnetizations in the direction of the easy magnetization axis. If a magnetic field is applied to the storage layer in the direction opposite to the easy magnetization axis (the direction of the small magnetizations), the magnetization of the ferromagnetic layers is reversed with the antiferromagnetic coupling being maintained. Since the magnetic lines of force are closed, the influence of the demagnetizing field is slight. Further, since the coercive force of each ferromagnetic layer determines the switching field of the storage layer, the reversal of magnetization with a small switching field can be accomplished.
As described above, it is essential in a magnetic memory to decrease the magnetic field (switching field) for reversing the magnetization of a storage layer and to improve the thermal stability. Accordingly, several shapes of the storage layer and the use of multi-layer structure including antiferromagnetic coupling have been proposed. However, it is known that in a minute ferromagnet included in a small magnetic memory cell, which is used in a highly integrated magnetic memory, e.g., a ferromagnet having a short axis with a width of submicrons to a few microns, a magnetic structure (edge domains) that is different from the magnetic structure of the central portion of the ferromagnet is generated at the end portions of the magnetization regions of the ferromagnet due to the influence of the demagnetizing force.
In a minute ferromagnet used in a memory cell of a highly integrated magnetic memory, the influence of edge domains appearing at its end portions is great, so that the change in magnetic structure pattern caused by the magnetization reversal becomes complicated. As a result, the coercive force and the switching field are increased.
In order to suppress the complicated change in magnetic structure as much as possible, the pinning of edge domains has been proposed (for example, see U.S. Pat. No. 5,748,524 and JP Laid-Open Pub. No.2000-100153).
Although it is possible to control the behavior of magnetization at the time of the magnetization reversal by pinning the edge domains, it is not possible to reduce the switching field. Further, since another structure must be added to pin the edge domains, this method is not suitable for a highly densified structure.